(1) Field of the Invention
This invention relates to a multi-layered vessel whose sectional structure of synthetic resin has at least a triple-layer used as a vessel for containing carbonated beverages or fruit juice.
(2) Description of the Prior Art
Blow or orientation blow molded plastic packing vessels are light in weight and have a falling strength but are inferior in heat resistance and gas barrier properties to glass. In the case where these vessels are used as vessels for beverages which refrain from permeation of fruit juice, oxygen, carbonic acid or the like which require to be filled at high temperature, a further improvement has been necessitated.
In view of the foregoing, a multi-layered vessel of at least a triple-layer has been developed in which within the resin (for example, polyethyleneterephthalate) to be used to form a vessel body is provided, as an intermediate layer, other resin which makes up for the disadvantage of the first-mentioned resin. As other intermediate layers, an ethylene vinyl alcohol copolymer having excellent gas barrier properties, polycarbonate polyamide having a heat resistance and the like are used.
This vessel having a multi-layered construction can be produced by blow molding or orientation blow molding an extruded or injection molded multi-layered parison in a manner similar to the case of a single-layer construction, and an intermediate layer formed therein is positioned in the midst or internally of the resin which forms a vessel body. In the vessel body in which the intermediate layer is positioned in the midst of the resin, there is present no difference in wall thickness between two layers, an inner layer and an outer layer, defined by the intermediate layer, but where the intermediate layer is positioned one-sided inwardly, there is present a difference in wall thickness between the inner and outer layers, in which case the inner layer is materially thin as compared with that of the outer layer.
The distribution of the wall thickness of layers in the multi-layered vessel is decided by the distribution of the wall thickness of a multi-layered bottom parison extruded or injection molded, and a proportion of the wall thickness of the parison will be a proportion of the wall thickness of a vessel without modification.
Molding of a multi-layered bottomed parison is effected by using a double nozzle composed of an outer flowpassage in communication with a nozzle orifice and an inner flowpassage opened in an extreme end of the outer flowpassage, as disclosed in U.S. Pat. No. 4,174,413, and a first resin forming a parison body and a second resin forming an intermediate layer are injected into a cavity through the outer flowpassage and inner flowpassage, respectively. While the wall thickness of the intermediate layer can be varied by the quantity of injection of the two resins, the intermediate layer is always one-sided inwardly, as a consequence of which the inner layer is formed into a thin layer.
The resins such as ethylene vinyl alcohol copolymer (EVOH), polyamide (FA) and the like increase in the quantity of permeation of oxygen, carbonic acid or the like as the moisture absorption increases. On the other hand, the biaxially oriented polyester resin used as bottles for carbonated beverages is lower in water vapor permeability than the aforementioned resin but the water vapor permeability is affected by the thickness as can be said generally in resins.
Therefore, in a multi-layered vessel in which an inner layer is formed in a small wall thickness, even if the resin forming a vessel is biaxially oriented polyethyleneterete, the gas barrier properties caused by the intermediate layer is materially decreased by the influence of the moisture absorption from the content, and the permeability of oxygen and carbonic acid increases. Therefore, such vessels are not suitable for use with beer, carbonated beverages and the like which are required to be stored for a long period of time, and in order to use such vessels as ones for food and drinks which refrain from permeation of oxygen, even a multi-layered vessels have to further increase its gas barrier properties.
Similarly, in order to enhance the heat resistance, even if a second resin as an intermediate layer comprising a resin having the heat resistance is injected, it is not favorable to make the wall thickness of the inner layer thin, and the inner layer is formed to be as thick as possible. However, so far a triple-layered heat resisting vessel in which an inner layer has a greater wall thickness than that of an outer layer has not been available.